This invention relates to a plug release apparatus used in the introduction and separation of fluids in a well, such as the introduction and separation of cement slurry and displacing fluid in an oil or gas well.
Cement is used in oil or gas wells for various purposes. One purpose is to secure a tubular string (e.g., a casing or a liner) in the well bore. This is typically done by pumping cement down the tubular string and forcing it back up an annular space between the outside of the string and the well bore or a larger diameter string in which the first-mentioned string is disposed. To separate the cement slurry from drilling mud typically in the well when the cementing operation begins, a bottom cementing plug is placed in line and pumped down the string by the force of the following cement slurry. This bottom plug serves to minimize contamination of the cement as it is being pumped down the tubular string. It also wipes any accumulated mud film from the inner diameter of the string and pushes it ahead. To separate a following displacing fluid used to push the cement slurry out the tubular string and up the annular space, a top cementing plug is placed in line and pushed down the string by the displacing fluid. This top plug follows the cement and wipes any accumulated cement film from the inner diameter of the tubular string. It also prevents or reduces any contamination of the cement by the displacing fluid.
In wells drilled on land, surface-mounted plug release apparatus are used in many cementing jobs to release the cementing plugs at the proper time. Normal job operations will have the bottom cementing plug loaded into the plug release apparatus prior to pumping cement. The top cementing plug will typically be loaded after the bottom plug is released
Subsea (ocean floor) completions are different from the aforementioned land-based cementing operations in that the cementing plugs used for separating the fluids are preferably located in the tubular string below the ocean floor. This is preferred because these plugs have a diameter large enough to wipe the inner diameter of the tubular string extending below the ocean floor, and this tubular string (and thus each plug) typically has a larger diameter than need be used for connecting this string with the equipment on the rig at the ocean's surface. Thus, the cement slurry is preferably pumped from the surface through a string of drill pipe smaller than the string being cemented, which smaller string extends between the surface rig and the downhole string to be cemented. Thus, there is a second type of plug container that houses elements, which may broadly be called “plugs” also, which are of smaller diameter to permit these plugs to pass through the narrower connecting string and into the downhole cementing plugs. A system using this technique is the Halliburton Energy Services' sub-surface release system (“SSR Cementing Plug Method”). This system provides a means of wiping different pipe sizes; therefore, smaller diameter drill pipe can be used as described instead of the larger diameter casing that otherwise would be run between the rig floor and the ocean floor.
Many offshore casing jobs run pipe from down-hole all the way up to the wellhead on the ocean floor. To do this, the casing is attached to drill pipe and lowered into position from the ocean surface. Liner jobs are extremely similar to the offshore casing job in that they use drill pipe to lower the liner into position except that the top of the pipe is not located at the well head on the ocean floor. Instead, the top of the pipe is located somewhere inside another casing string below the ocean floor or below the surface on a land-based job. The plug set used on an offshore casing job is referred to as an SSR. The liner plug set has a similar configuration and can be run offshore or on land.
These drill pipe operated SSR or liner cementing plugs sometimes do not operate as designed. In some instances, this can (1) prevent cement from being displaced from the liner or casing, (2) cause the hanger to prematurely set, and/or (3) cause rupture disks on tools above to deploy prematurely. This results in a well bore full of cement or over displaced cement. To remedy this, drill out, drill around, bull heading, or retrieval of the liner may be needed, all of which are very costly. While the exact cause of the problem is not known, some possibilities include the following: debris interfering with the releasing mechanism in the plug releasing mechanism; obstacles such as shoulders in the drill pipe or tools that positively stop the latch-down plug; or hydraulic lock between the latch-down plug body and the releasing mechanism.